Reliable High-Speed Short-Range Underwater Wireless Optical Communication Systems

Abstract

In recent years, the high demand for high-speed communications at short-range applications motivates underwater wireless optical communication (UWOC) systems to be an alternative technology rather than acoustic and radio frequency (RF) technologies. However, UWOC systems require alignment, which is challenging underwater due to currents and waves of seawaters in addition to the system mobility. The speed of UWOC links is restricted in practice due to the narrow bandwidth of opto-electronic components and scattering in seawaters. In addition, the transmitted optical power is restricted by noise and limited transmitted power due to eye-safety standards. In order to tackle these challenges and provide reliable high-speed links, this thesis proposes three new UWOC approaches which are appropriate for point-to-point and broadcast communications. We propose angular multiple-input multiple-output (A-MIMO) and tracking AMIMO (TA-MIMO) communication systems for point-to-point links. In the first part of this thesis, A-MIMO systems are proposed and modeled rather than conventional MIMO (C-MIMO) systems. Unlike C-MIMO systems, A-MIMO systems send information in angle rather than in space, thus relaxing the strict requirements of on axis alignment and fixed channel length are relaxed. The main features of A-MIMO systems are highlighted, and maximum link lengths and angle-of-arrival (AoA) distributions are derived. C-MIMO and A-MIMO systems are simulated using a Monte Carlo numerical ray tracing (MCNRT) method. Numerical results indicate that AMIMO systems are more robust than C-MIMO systems. As well, A-MIMO systems can be implemented with smaller sizes. In the second part of this thesis, motivated by the performance of A-MIMO systems, we introduce TA-MIMO systems. TA-MIMO systems inherit tracking advantages from their optical structures by which they infer the relative displacement and tilt between ends of the link. Compared to A-MIMO systems, TA-MIMO systems further enhance the link against tilt misalignment, and they perform localization functions besides communication. The architecture of TA-MIMO systems is described by highlighting their inherent tracking advantages. Comprehensive analytic models for TA-MIMO and A-MIMO links are derived by considering link misalignment, channel impairments, and receiver noise. Closed-form expressions for AoA distributions are derived and verified using a MCNRT method. Utilizing the architecture of TA-MIMO systems, a pointing, localization, and tracking (PLT) scheme is proposed and modeled. Numerical results indicate that TA-MIMO systems outperform A-MIMO and C-MIMO systems when the misalignment is presented by both displacement and tilt. The third part of this thesis proposes a novel sea ice diffusing optical communication (SDOC) system for reliable broad-band broadcast communications under sea ice, such as in the Arctic and Antarctic zones. SDOC systems utilize the sea ice sheets floating on the sea surface to diffuse optical beams with wide spots and omnidirectional patterns from the transmitter to receivers. SDOC channels are modeled as seawater-sea ice cascaded layers (SSCL) in which the vertical channel is divided into multiple layers based on their optical characteristics. An efficient methodology is proposed to compute channel impulse responses (CIRs), ensuring accuracy and reducing computing time. In order to overcome the limitations of channel and receiver noise, we propose a system architecture that enhances system speeds and ranges. Numerical results reveal that, under a snow-covered sea ice sheet with a thickness of 36 cm, the proposed system can achieve a communication speed of 100 Mbps with ranges up to 3.5 meters with BER less than 10−3 and average transmitted power of 100 mW. This work serves as a design guide to broadband-broadcast communications under the frozen oceans. For example, a group of mobile sensors navigating below sea ice sheets in Arctic regions could use SDCOC systems for real-time signaling exchange.